Tag candidate query was not selecting tags attached to zero bugs.

Follow-up to commit e41119d, a2294e9, and 8ed85ce6.

core/tag_api.php

@@ -413,8 +413,11 @@ function tag_get_candidates_for_bug( $p_bug_id ) {
 	$t_params = array();
 	if ( 0 != $p_bug_id ) {
 		$t_bug_tag_table = db_get_table( 'mantis_bug_tag_table' );
-		$query = "SELECT t.id, t.name, t.description FROM $t_tag_table t
-				WHERE t.id IN ( SELECT tag_id FROM $t_bug_tag_table WHERE bug_id != " . db_param() . ' )';
+		$query = "SELECT id, name, description FROM $t_tag_table WHERE id IN (
+				SELECT t.id FROM $t_tag_table t
+				LEFT JOIN $t_bug_tag_table b ON t.id=b.tag_id
+				WHERE b.bug_id IS NULL OR b.bug_id != " . db_param() .
+			')';
 		$t_params[] = $p_bug_id;
 	} else {
 		$query = 'SELECT id, name, description FROM ' . $t_tag_table;